Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE (Release 8).
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Channel Quality Report in LTE (Release 8)
Channel quality information is used in a multi-user communication system to determine the quality of channel resource(s) for one or more users. This information may be used to aid in a multi-user scheduler algorithm of the eNodeB (or other radio-access elements such as a relay node) to assign channel resources to different users, or to adapt link parameters (e.g. modulation scheme, coding rate, or transmit power) so as to exploit the assigned channel resource to its fullest potential.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example discussed in the “Long Term Evolution” work item of 3GPP, the smallest unit of resources that can be assigned/allocated by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 3. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.7.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference). In the ideal case, channel quality information for all resource blocks for all users should be always available to the scheduler so as to take an optimum scheduling decision. However, due to constrained capacity of the feedback channel it is not possible/feasible to ensure this type of up-to-dateness of channel quality information. Therefore, reduction and/or compression techniques are required so as to transmit—for example—channel quality information only for a subset of resource blocks for a given user. In 3GPP LTE, the smallest unit for which channel quality is reported is called a sub-band, which consists of multiple (n) frequency-adjacent resource blocks (i.e. n·NBWRB subcarriers).
Channel Quality Feedback Elements
In 3GPP LTE, there exist three basic elements which may or may not be given as feedback for the channel quality:                Modulation and Coding Scheme Indicator (MCSI), which is also referred to as Channel Quality Indicator (CQI) in the 3GPP LTE specifications,        Precoding Matrix Indicator (PMI) and        Rank Indicator (RI)        
The MCSI suggests a modulation and coding scheme that should be employed for downlink transmission to a reporting user equipment, while the PMI points to a precoding matrix/vector that is to be employed for multi-antenna transmission (MIMO) using an assumed transmission matrix rank or a transmission matrix rank that is given by the RI. Details on channel quality reporting and transmission mechanisms are can be found in 3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”, version 8.7.0, sections 5.2 and 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”, version 8.7.0, section 7.2 (all documents available at http://www.3gpp.org and incorporated herein by reference).
All of these elements are summarized as under the term channel quality feedback herein. Hence, a channel quality feedback can contain any combination of or multiple MCSI, PMI, RI values. Channel quality feedback reports may further contain or consist of metrics such as a channel covariance matrix or elements, channel coefficients, or other suitable metrics as apparent to those skilled in the art.
Triggering and Transmission of Channel Quality Feedback
In 3GPP LTE (Release 8) there are different possibilities defined, how to trigger the user equipments to send channel quality feedback on the downlink channel quality. Besides periodic CQI reports (see section 7.2.2 in 3GPP TS 36.213, version 8.7.0), there is also the possibility to use L1/L2 control signaling to a user equipment to request the transmission of the so-called aperiodic CQI report (see section 7.2.1 in 3GPP TS 36.213, version 8.7.0). This L1/L2 control signaling can also be used in the random access procedure (see section 6 in 3GPP TS 36.213, version 8.7.0, incorporated herein by reference). In both these cases, a special CQI request field/bit/flag is included in the control message from the eNodeB/relay node.
The L1/L2 control signaling that conveys information about an Uplink assignment is sometimes called UL-DCI (Uplink Dedicated Control Information). FIG. 4 shows an example of the DCI format 0 for FDD operation as defined in 3GPP TS 36.212, section 5.3.3.1.1 which serves to convey uplink DCI (please note that the CRC field of DCI format 0 is not shown in FIG. 4 for simplicity. The CQI request flag contains information whether the receiver should transmit CQI within the allocated uplink resources or not. Whenever such a trigger is received, the user subsequently transmits the feedback generally together with uplink data on the assigned Physical Uplink Shared CHannel (PUSCH) resources (the detailed procedure is described in section 7.2 et seq. in 3GPP TS 36.213, version 8.7.0).
Further Advancements for LTE—LTE-Advanced (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07) in November 2008. Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved which is also referred to as “Release 10”. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
In order to extend the overall system bandwidth, LTE-A (Release 10) uses carrier aggregation, where two or more component carriers are aggregated in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation. It is commonly assumed that a single component carrier does not exceed a bandwidth of 20 MHz.
A terminal may simultaneously receive and/or transmit on one or multiple component carriers depending on its capabilities:                An LTE-Advanced (Release 10) compatible mobile terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers. There is one Transport Block (in absence of spatial multiplexing) and one HARQ entity per component carrier.        An LTE (Release 8) compatible mobile terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Release 8 specifications.        
It is also envisioned to configure all component carriers LTE (Release 8)-compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are same. Consideration of non-backward-compatible configurations of LTE-A component carriers is not precluded.
Channel Quality Feedback in LTE-A (Release 10)
As there is only one component carrier defined in LTE (Release 8), there is no ambiguity at the user equipment on which portion of the system bandwidth CQI reporting is to be done. The CQI request flag (together with the current transmission mode) is unambiguously indicating to the user equipment how to provide CQI feedback to the eNodeB.
With the introduction of carrier aggregation in LTE-A (Release 10) and assuming that the LTE (Release 8) CQI reporting procedures should be reused, there are different possibilities how a CQI request can be interpreted by the user equipment. As shown in FIG. 5, it may be generally assumed that UL-DCI (containing the CQI request) for uplink transmission that is transmitted from a eNodeB or relay node to a user equipment is placed within a single downlink component carrier. A simple rule to handle the CQI request at the user equipment would be that whenever a UL-DCI requests a CQI transmission by the user equipment, same applies to the downlink component carrier where the corresponding UL-DCI is transmitted. I.e. the user equipment would only send aperiodic CQI feedback in a given UL transmission for those downlink component carriers that comprised a UL-DCI requesting a CQI report at the same time.
An alternative handling of UL-DCI comprising a CQI request is shown in FIG. 6. Whenever a UL-DCI requests a CQI transmission by the user equipment, the user equipment applies said request to all downlink component carriers available for downlink transmission to the user equipment.
When downlink transmission can occur on multiple component carriers, an efficient scheduling and link adaptation depends on the availability of accurate and up-to-date CQI. However, in order to make efficient use of the control signaling and CQI transmission resources, it should be possible to control for how many and which component carriers a CQI is to be requested (from the network side) and transmitted (from the terminal side).
According to the first solution discussed above with respect to FIG. 5, in order to request CQI for multiple component carriers the number of component carriers for which CQI is requested is identical to the number of required transmitted UL-DCI messages. In other words, to request CQI for five component carriers it is required to transmit five times more UL-DCI messages than for the case of requesting CQI for just a single component carrier. This solution is therefore not very efficient from a downlink control overhead point of view. According to the second solution above illustrated in FIG. 6, a single uplink DCI message requests CQI for all component carriers. Therefore the downlink control overhead is very small. However, the resulting uplink transmission always requires a large amount of resources to accommodate the transmission of CQI for all component carriers, even though the network knows that it currently requires CQI only for a single selected component carrier. Therefore this is not efficient for the usage of uplink resources, and does not offer any flexibility for the number of requested component carrier CQI.